In many filtration applications, a filtration device may include a filter element, such as a septum, and a filter-aid material. The filter element may be of any form such that it may support a filter-aid material. For example, the filter element may include a cylindrical tube or wafer-like structure covered with a plastic or metal fabric of sufficiently fine weave. The filter element may be a porous structure with a filter element void to allow material of a certain size to pass through the filtration device. The filter-aid material may include one or more filtration components, which, for example, may be inorganic powders or organic fibrous materials. Such a filter-aid material may be used in combination with a filter element to enhance filtration performance.
For example, the filter-aid material may initially be applied to a septum of a filter element in a process known as “pre-coating.” Pre-coating may generally involve mixing a slurry of water and filter-aid material, and introducing the slurry into a stream flowing through the septum. During this process, a thin layer, such as, for example, about 1.5 mm to about 3.0 mm, of filter-aid material may eventually be deposited upon the septum, thus forming the filtration device.
During filtration of a fluid, various insoluble particles in the fluid may become trapped by the filter-aid material. The combined layers of filter-aid material and particles and/or constituents to be removed accumulate on the surface of the septum. Those combined layers are known as “filter cake.” As more and more particles and/or constituents are deposited on the filter cake, the filter cake may become saturated with debris to the point where fluid is no longer able to pass through the septum.
To combat this situation, a process known as “body feeding” may be used. Body feeding is the process of introducing additional filter-aid material into the fluid to be filtered before the fluid reaches the filter cake. The filter-aid material will follow the path of the unfiltered fluid and will eventually reach the filter cake. Upon reaching the filter cake, the added filter-aid material will bind to the cake in a similar manner to how the filter-aid material is bound to the septum during the pre-coating process. The additional layer of filter-aid material may cause the filter cake to swell and thicken, and may increase the capacity of the filter cake to entrap additional debris. The filter aid typically has an open porous structure, which maintains an open structure in the filter cake, thus ensuring continued permeability of the filter cake.
In the field of fluid filtration, diatomaceous earth and natural glasses may be employed as filter aids. Diatomaceous earth products may be obtained from diatomaceous earth (also called “DE” or “diatomite”), which is generally known as a sediment-enriched in biogenic silica (i.e., silica produced or brought about by living organisms) in the form of siliceous skeletons (frustules) of diatoms. Diatoms are a diverse array of microscopic, single-celled, golden-brown algae generally of the class Bacillariophyceae that possess an ornate siliceous skeleton of varied and intricate structures including two valves that, in the living diatom, fit together much like a pill box.
Diatomaceous earth may form from the remains of water-borne diatoms and, therefore, diatomaceous earth deposits may be found close to either current or former bodies of water. Those deposits are generally divided into two categories based on source: freshwater and saltwater. Freshwater diatomaceous earth is generally mined from dry lakebeds and may be characterized as having a low crystalline silica content and a high iron content. In contrast, saltwater diatomaceous earth is generally extracted from oceanic areas and may be characterized as having a high crystalline silica content and a low iron content.
Natural glasses, commonly referred to as “volcanic glasses,” are generally formed by the rapid cooling of siliceous magma or lava. Several types of natural glasses are known, including, for example, perlite, pumice, pumicite, obsidian, pitchstone, and volcanic ash. Prior to processing, perlite may be gray to green in color with abundant spherical cracks that cause it to break into small pearl-like masses. Perlite may thermally expand upon processing. Pumice is a lightweight glassy vesicular rock. Obsidian may be dark in color with a vitreous luster and a characteristic conchoidal fracture. Pitchstone has a waxy resinous luster and may be brown, green, or gray. Volcanic glasses such as perlite and pumice occur in massive deposits and find wide commercial use. Volcanic ash, often referred to as “tuff” when in consolidated form, may include small particles or fragments that may be in glassy form.
In the field of filtration, methods of particle separation from fluids may employ diatomaceous earth products or natural glass products as filter aids, The intricate and porous structure unique to diatomaceous earth may, in some instances, be effective for the physical entrapment of particles in filtration processes. It is known to employ diatomaceous earth products to improve the clarity of fluids that exhibit “turbidity” or contain suspended particles or particulate matter. “Turbidity” is the cloudiness or haziness of a fluid, where the haze may be caused by individual particles that are suspended in the fluid. Materials that may cause a fluid to be turbid include, for example, clay, silt, organic matter, inorganic matter, and microscopic organisms.
Diatomaceous earth and natural glass may be used in various aspects of filtration. For example, as a part of pre-coating, diatomaceous earth or natural glass products may be applied to a filter septum to assist in achieving, for example, any one or more of: protection of the septum, improvement in clarity, and expediting of filter cake removal. As a part of body feeding, diatomaceous earth or natural glass may be added directly to a fluid being filtered to assist in achieving, for example, either or both of: increasing flow rate and extending of the filtration cycle. Depending on the requirements of the specific separation process, diatomaceous earth or natural glass may be used in multiple stages including, but not limited to, in a pre-coating stage and in a body feeding stage.
Known diatomaceous earth or natural glass products may suffer from any number of attributes that make them inappropriate for filtration use, cause them to be less desirable, or cause them to have poor or improvable performance in a particular application, for instance in filtering applications. For example, known diatomaceous earth or natural glass products may have at least one of a high crystalline silica content, a high impurity content, and low permeability. Thus, it may be desirable to improve diatomaceous earth or natural glass products such that they exhibit improved performance in a given application, such as lower impurity content and/or higher permeability in filtration applications.